1. Field of the Invention
The present teachings relate to photolithography systems and methods such as for use in fabricating semiconductor devices.
2. Description of the Related Art
Conventional “far-field” photolithography systems use light and a lens system to image a reticle having a pattern thereon onto a layer of photosensitive material deposited on a semiconductor wafer. Such conventional photolithography systems are referred to as “far-field” systems because the image produced is in the far field. Accordingly, the size of features in the reticle pattern that may be reproduced on the layer of photosensitive material is limited by far-field diffraction. The minimum feature size W that may be produced using a conventional far-field photolithographic system is
                    W        =                              k            1                    ⁢                      λ            NA                                              (        1        )            where NA is the numerical aperture of the system, λ is the wavelength of light used by the photolithography system, and k1 is the resolution factor, which depends on other aspects of the system including, for example, the aberrations introduced by the specific photolithography system and the properties of the photosensitive material. According to this equation, to produce smaller features, the photolithographic system must utilize smaller resolution factor, a larger numerical aperture, a smaller operating wavelength, or a combination thereof.
Current far-field lithographic systems include complex lens that are well-corrected for aberrations. Accordingly, current far-field lithographic techniques have decreased the resolution factor to k1≈0.3, which is slightly greater than the theoretical lower limit of 0.25 for half-pitch imaging. These complex lens, however, may include many optical elements and are expensive.
The numerical aperture has also been increased. However, in systems where light propagates through air from the lens system to the semiconductor wafer having photosensitive material thereon, the numerical aperture is limited to one. Immersion lithography wherein the light propagates through a medium having an index of refraction greater than one has lead to increases in NA. Immersion techniques, however, suffer from problems such as incompatibility of the fluid and the wafer, bubble formation, and polarization effects. Further increases in NA are limited, however, because of the limited range of compatible immersion fluids having refractive indexes above one.
Smaller features may also be produced by using light sources having shorter operating wavelengths. Commercial photolithography systems may use visible light having wavelengths in the range of 350 nm-800 nm. Ultraviolet photolithographic systems operating with wavelengths in the range of 100 nm-350 nm may be used to print smaller features. Ultraviolet systems, however, also suffer from drawbacks such as increased cost, shorter lamp lifetimes, and lower efficiency. Furthermore, photoresist that is sensitive to visible light is cheaper and more robust with respect to airborne contaminants than ultraviolet-sensitive photoresist.
Thus, what is needed are photolithography systems that are not restricted to the diffraction limit of far-field optical systems and need not rely on use of complex and expensive lenses, immersion techniques, or the use of ultraviolet wavelengths.